wheel/tire size usable w/ my front suspension?

[mattfarmer]

Hey guys, I have a question about wheel/tire size & front suspension...

I'm almost ready to order wheels & tires, my baja has a +6" wider beam & +4" wider trailing arms. I'd really like to run Empi Race Trim beadlocks, but I heard that if I run 7" wheels it'll beat my front end to death...

I was told I should run 4" wheels in front with 31x10.50R15's.... That sounds dangerous to me, way to narrow for a 10.50 tire on the street... As of right now I'm just going to run 7" wheels all the way around with 31x10.50's
....any advice?

[dustymojave]

While most street tire vendors and shops will tell you that a 4" rim is way too narrow fro a 10.50 width tire, the offroad racers in Class 5-Open, Class 10, 12 and 30 almost ALL race on that combination.

One of the main considerations in choosing that combination is the tire contact patch offset if you use 7" rims. That increased scrub radius places a lot of leverage on all of the steering components, spindles, tie rods, tie rod ends, steering box, all the way to your own hands. You can overcome a lot of the load involved in that by using power steering, but that then increases tendency to bend and break parts between the ground and your hands. The narrower rims put the contact patch at least close to where it SHOULD be. Any alignment guy worth the title would be able to tell you the handling issues related to that, but those guys are mighty few and far between. Further...I have NEVER observed or even heard of any danger related to 10.50 tires mounted on 4" wide rims. Nor has anyone been able to explain WHY it would be dangerous to use such a combination. Such fuss seems to be totally unscientific old wives tales. Is there any more "danger" related to the narrow rims than to using tires rated for 2,200lbs per tire on a 2,000lb car?
_________________
Richard
Offroading VW based cars since 1965
Tech Inspection 1963 - 2012 SCCA/SCORE/HDRA/MORE/MDR +
Retired from building Bajas, Fiberglass Buggies and Rails in the Mojave Desert. Also Sprints & Midgets, Dry Lakes, Road Race cars. All types New and Vintage
SoCalBajas Member
Kicked Cancer's A$$...1st and 2nd round...Fight ain't over yet.

[dirtslinger]

Use a 4" rim and run a 9.50 or 10.50 tire. You'll have no issues.

[mattfarmer]

Aside from the mickey thompson baja tires & super swampers I've had a really hard time finding and 31x9.50r15 tires... even after looking in the wheel & tire thread I can't find anything... any suggestions?

[dustymojave]

Try 30 x 9.50 or 31 x 10.50.
_________________
Richard
Offroading VW based cars since 1965
Tech Inspection 1963 - 2012 SCCA/SCORE/HDRA/MORE/MDR +
Retired from building Bajas, Fiberglass Buggies and Rails in the Mojave Desert. Also Sprints & Midgets, Dry Lakes, Road Race cars. All types New and Vintage
SoCalBajas Member
Kicked Cancer's A$$...1st and 2nd round...Fight ain't over yet.

[KrAzY-BaJa]

I ran what your setup is, no problems and keeps it all the same.
7" btr's and 31s
I wouldnt get beadlocks though its just more weight
_________________
70% of what you buy is something to start with.
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[Mr_Zed_ZX20]

Ok, here is my beef with the whole scrub radius issue. I agree with everything posted above in regards to the poor handling, increased wear, etc. So why is it that nobody makes a wheel with a more respectable backspacing to place the center of the tire closer to the axis of the king pin? The best I've seen is a Centerline 15x5" wheel with 3.375" backspacing or a 15x3.5" with a 3.5" backspacing. The Empi Race Trim, BTR, Method, and others are 1.75" backspacing for a 4" wide wheel. I get that having a wide track width is good, but is it worth more than good steering geometry?

On my current setup I'm running a 6in wider beam w/ 10" coilovers, Tweeds 4" x 1.25" arms, Combo spindles, and Jamar pro-X brakes with a BTR 15x4" wheel and yokohama 742s tire (30"dia.) I haven't taken exact measurements but dropping a ruler along the kingpin axis to the ground and eyeballing the distance to the center of the tire looks to be almost 6 inches of scrub Gee, and I was pinging over adding 10mm of scrub to my front when I did the Mendeola S2/porsche brake conversion on my street car, lol.

Something else I wanted to add for those who aren't aware of how much of a difference can be made with a properly placed wheel in regards to steering effort. On my German looker I had a 2in narrowed BJ beam, cb 2.5 drop spindles, and centerline wheels 15x6 with 3.5" backspacing, and 185/60/r15 tires (24 dia.). The steering effort was considerable and the steering was crappy to say the least. I also had issues with my tires hitting the back of my headlight buckets at full lock. Now with the Mendeola S2, Porsche 18x8" with 50mm offset (approx 6 backspacing) and 225/40/r18 (25 dia.) the steering is almost effortless even with 1.5" wider and 1" taller tire. And no more headlight bucket rubbing (wheels are actually set in further than the 15's were too). I realize this isn't a real scientific analysis, but I hope it gives you an idea of how just a few inches in tire placement makes a big difference in steering effort and effectiveness.

[mattfarmer]

I was wondering te same things about wheel manufacturers and why no one has better, or more backspacing....

[Mr_Zed_ZX20]

To address the wide tire/skinny wheel issue. I believe the reason tire shops as well as the tire mfr's will say a 10.5" tire is too small for a 4" wide wheel is due to the crowning that will occur on the tread when the beads are pushed that close together. Of course the taller the sidewall the less this becomes a problem, but for a street driven car I can understand why they would steer an individual away from such a practice. The last thing anybody wants is a whiner complaining about funky tire wear or a slimy lawyer shaving another reason to sue someone. I personally like the way a manx style buggy handles with a "pinched" tire as it reduces the contact patch and makes the car less likely to hunt back and forth in the asphalt grooves than a flat wider tire. <<I'm speaking on street driving experience only.